As part of our continuing interest in understanding the molecular-genetic basis of cell-cell interactions, we have extended our study of the structure and expression of the genes which encode the major histocompatibility antigens and the immune response antigens of the mouse. Over the past year we have exploited recombinant DNA techniques, in concert with DNA-mediated gene transfer methods, and more classical serological and cell-mediated immunological approaches to evaluate the function of cloned H-2 and I-A antigen genes.